The description of references in this Section is not intended to constitute an admission that any patent, publication or other information referred to herein is “Prior Art” with respect to the Present Invention, unless specifically designated as such.
HST systems are well known in the industry, and are more fully described in, e.g., U.S. Pat. No. 5,314,387, the contents of which are incorporated herein by reference in its entirety. In general, a typical HST system possesses, inter alia, a hydraulic pump and a hydraulic motor mounted in a housing. The pump and motor are hydraulically-linked through a generally-closed circuit, and both consist of a rotatable body with pistons mounted therein. A hydraulic fluid, such as, for example, oil, is maintained in the generally-closed circuit, and the HST generally has a sump, or reservoir, with which the generally-closed circuit can exchange oil. In certain instances, this sump may be formed by the housing itself.
The pump is usually driven by an external motive source, such as, for example, pulleys and belts or drive shafts connected to an internal combustion engine. The pump pistons engage a moveable swash plate and, as the pump is rotated by an input source driven by the external engine, the pistons engage the swash plate. Other HST designs may use a radial piston or ball piston pump and motor design, but the general operation is, in any event, similar, and the Present Invention is not limited to use with a specific design. Movement of the pump pistons creates movement of the hydraulic fluid from the pump to the motor, causing the rotation thereof. The motor pistons are engaged against a fixed plate, and rotation of the motor drives an output shaft engaged thereto. This output shaft may be linked to mechanical gearing and output axles, which may be internal to the HST housing, as in an integrated hydrostatic transaxle (“IHT”), or external thereto.
The system is fully reversible in a standard HST. This means that as the swash plate against which the pump pistons move is moved, the rotational direction of the motor can be changed, such as in a forward or reverse direction. In addition, there is a “neutral” position where the pump pistons are not moved in an axial direction, so that rotation of the pump does not create any movement of the hydraulic fluid.
The HST generally-closed circuit has two sides, namely a high pressure side in which oil is being pumped from the pump to the motor, and a low pressure, or vacuum, side, in which oil is being returned from the motor to the pump. When the swash plate angle is reversed, the flow out of the pump reverses so that the high pressure side of the circuit becomes the vacuum side, and vice versa. This hydraulic circuit can be formed as porting formed within the HST housing internal to a center section on which the pump and motor are rotatably mounted or in other ways known in the industry. Check valves are often used to draw hydraulic fluid into the low pressure side to make up for fluid lost due to leakage, for example. Such check valves may be located so that they directly contact the porting or they may be located separate from the porting and connected through additional bores to the closed circuit.
There is a need to have a means to open, or bypass, this closed circuit in certain circumstances. For example, when the vehicle is stopped, the oil in the closed circuit provides for hydraulic braking, making it difficult to manually move the vehicle. Mechanical bypass designs are known in the art and are described in, for example, U.S. Pat. No. 5,423,182, the contents of which are incorporated herein by reference in its entirety. Such designs generally achieve bypass by opening the closed hydraulic circuit to the sump by, e.g., opening check valves in the circuit or by opening a shunt between the high pressure and low pressure sides of the circuit. Such designs are generally complicated and add significantly to the cost of the unit.
In order to effect the actuation of the bypass in a more cost-effective manner, a bypass actuator and a bypass arm are often provided. A linkage is then connected to the actuator or the arm to operate the bypass mechanism. Such linkages are generally attached to some portion of the vehicle by a vehicle manufacturer, entailing complexity in their assembly process by requiring an attachment or other interface location, as well as the need for assembly or connection of the linkage. Thus, there is a need for an improved bypass linkage.